Existing Oil Boiler as Hydraulic Separator

[Clarkbug]

Hello All.

So my install is finally about to go somewhere. Hurrah!

I had my installer over today to talk shop, and he was looking over my schematics, and my existing system. Turns out to pipe this up the way I have drawn, that really involves taking apart all of my existing near-boiler piping and re-doing all of it. This is not what I was hoping for, but I think I also turned a blind eye to it in the hopes that this wouldnt be the case. Just proves that ignoring something does not make it go away.

At any rate, what he was proposing was to utilize a set of tappings on the oil boiler to flow my water from storage through this boiler and then back to storage when there is a call for heat. This way the configuration of my zones and circs wont have to change, and it gets me out much cheaper as far as labor goes.

I know keeping more iron hot is going to cost me efficiency, and there will be some heat loss out the chimney this way, but does anyone have any idea what kind of losses this would create? Im trying to determine if I want to bite the bullet and have it all re-piped, or if I want to just flow through the OB and be happy. Both will work, just the engineer in me wants it to look like the perfect schematic.

Thoughts?

 

[bpirger]

When I get my new plumbing done, I'll be able to tell you this directly. Right now I have my oil boiler in my primary loop, but I am repiping and will set things up so I can either keep it IN the primary loop or have the oil boiler as a secondary loop....so I can switch a few valves and tell ya. Of course that doesn't help. And I have a Riello OC-3 burner with the direct vent stack, i.e. no tall chimney. My oil boiler sits outside in its' own little shed with the oil tank. No basement, and I didn't want to hear the roar of the boiler....so I put it outside.

I wanted to keep the OB in the loop and warm so I wouldn't have to worry about it for backup. But I haven't turned it on since 17NOV10, over a year, so maybe that's dumb! If I take it out of the primary loop, then I will have to have some freeze protection on it. I will also have to add a circulator so when it fires, it injects the hot water into the primary loop. For me, this should be simple, as my oil boiler has the Honeywell control that has the circulator taps whenever it is fired.


I don't know your piping, but I'd think it might be straight forward to have the oil boiler feed into your existing primary loop as a secondary. Presumably this is how your boiler is being set up. Then either the WB or the OB can heat up the primary loop, and it doesn't matter from where the heat comes. I'd think it'd be somewaht easy to pipe that up...but who knows.

I have a controller looking at the Garn water and if it is less then X degrees, it will send power to the oil burner, so the call for heat will then fire the oil boiler. Otherwise, the Garn supplies the hot water....So it is either or, and I can set the temp for when to fire the oil. Of course, it isn't all setup yet....

How tall is your chimney? How often does your oil boiler fire without a load call just to keep warm? Maybe that will give you an idea of how much you lose. How warm is the stack going to the chimney when not firing? Maybe you can read the temp in the stack? I've thought about that on mine too....though never have!

 

[ewdudley]

Tough call. If you have a DHW coil in the existing boiler, that could make your decision easier. Newer guns and boilers don't flow much if any air when they're not running, so that could mitigate heat loss. And it would be great to have the existing boiler hot and dry as it sits there on standby for the next few decades. And losing heat into a dampish area might be nice.

Check out the mattd860 'figure-eight' hookup that gregv used, pumping hot wood boiler water into the top of the existing boiler is the way to go. He got his system up in a week, boiler, flue, plumbing, done.

https://www.hearth.com/econtent/index.php/forums/viewthread/81444/P30/

 

[Clarkbug]

Thanks as always for the quick replies gents.

My chimney is about 32' tall, there is a baro damper on it now. Its a cold start boiler, so it only fires when there is a load, thats all. No DHW currently connected to anything, but that will be another zone setup in the future when funds allow....

EW, the thought my boiler guy had was exactly the "Figure 8" setup from mattd860, with the return that my circs are on the return side and not the supply side. The use of tappings is just the alternative to tapping into the maifolds, but I would think from a flow standpoint its the same.

It is my basement, and its pretty darn dampish there.

My leaning is towards leaving the piping as-is, and just flowing through the boiler. Then when something has to be replaced, I can go ahead and re-pipe it all. For now I just dont know if I can spend that extra money, thats all. I just was hoping to justify it to my brain one way or another...

 

[maple1]

Clarkbug, I am very interested in how you get your system set up. A Varm 40 is at the top of my planning list, I've already been talking with Smokeless Heat (actually, I think the plumbing hints he gave me to date might be from some work he did with you), I've got my eye on a pile of 330 gallon propane tanks, and I will be getting a new cold start oil boiler at the same time. I was talking once to my install guy ( I ran into him somewhere else) - he quickly thought running thru the oil boiler would be the easiest. But he's never seen anything like a Varm gasser hooked to propane tanks before. Looking forward to more details when you get things up & running.

 

[Clarkbug]

Just to bring this thread back to life again and try to get some info....

So some of my piping is done.....and it turns out that the guy has the supply from my storage tanks connected to the return side of the OB.

What Im realizing is that as soon as the cold water from the house gets back to the boiler, its going to temper my nice hot water from the storage tanks. Then on the other side, its going to take that mix and try to dump it back to my storage tanks.

Should I have the pipes switched so that hot water from storage goes directly to the loads in the house, and then returns back to the storage tanks? The OB would still be a hydraulic separator for when the flows arent equal, but it would prevent the tempering that Im a little worried about....

 

[in hot water]

A hydraulic separator, in the true sense, would be a four port device. It could be a tank or a boiler if the ports are located such that any pressure drop is eliminated, or all but eliminate. This assures that the circulators on either side are not effecting one another. Unless the flows are exactly the same on both sides of the hydraulic separator, there will be some temperature blending going on inside.

In your case I think you are looking to add a buffer, or some mass to the system, not actually a separator. And that may be fine for what you are trying to accomplish.

Here is a cut away of a hydraulic sep in action.

hr

 

[hobbyheater]

I know keeping more iron hot is going to cost me efficiency, and there will be some heat loss out the chimney this way, but does anyone have any idea what kind of losses this would create?

Thoughts?

A cast-iron slant fin oil boiler (295 lbs) with a 1.10 nozzle would run 3 1/2 hours in 24 hours to maintain standby reserve temperature. That represents a heat loss of 350,000 BTUs or about 50 lbs of wood in 24 hours.

Separating the oil boiler at this time is going to be a big expense if you have to pay someone to do it, but in the long term it would be a better system.

 

[Clarkbug]

Thanks for the replies.

I guess I should mention that Im not really using the OB as a separator, but its going to function as one, somewhat.

Im at work, so I cant readily post a schematic of what my system currently looks like, and what I had planned. It was going to follow the PT-1 diagram from Tarm, and then my installer suggested just using the spare tappings on my existing OB as a cost savings measure. Thats what I had posted about initially, and it seemed OK to my brain. This is where it would function as a separator, since my storage circ loop would be hydraulically independent from my load circs, and any mis-match in flow would be addressed inside the boiler.

Instead it turns out that the tappings were not used, and a T was installed in my supply and return leaving the boiler. However, the hot supply from my storage was piped to the cold return from my loads, and vice versa. So essentially my hot water from the wood boiler/storage would immediately be tempered with cold water returning from the house, and that makes me sad.

So I have a call into the guy to swap the pipes around, since thats not what we had discussed, and he isnt done piping things anyway.

I do have to pay someone to do it at this point, since the whole intent was for it to be done months ago. If I had started on it back this summer, I probably could have it done by now all on my own, but I just wanted someone that had done this before to take care of it. I agree that long term it would be much better to avoid flow through my OB, but short term I cant afford that, so Ill just try and get it running reasonably well.

Also, since my boiler isnt cast iron and doesnt need to maintain a temp, Im hoping that it wont have quite those losses while Im not using it....

 

[huffdawg]

It won't be heatloss if its in a living space . Did you say it was in your basement.

 

[ewdudley]

A cast-iron slant fin oil boiler (295 lbs) with a 1.10 nozzle would run 3 1/2 hours in 24 hours to maintain standby reserve temperature. That represents a heat loss of 350,000 BTUs or about 50 lbs of wood in 24 hours.

Doesn't really sound right, not at all.

Back when oil was less than two dollars a gallon we ran an oil boiler all summer just to keep the boiler hot and dry and to supply DHW. I kept pretty good records and we burned less than a gallon a day, which is still a lot, but it's a whole lot less than three and half or four gallons a day

 

[Clarkbug]

Indeed it is in my basement. I was thinking of heat loss up the chimney.

Just spoke with the installer, and he mentioned that he piped it this way deliberately. The reason being that if piped the opposite way, it would push through the flow checks and potentially heat all zones of the house when only one is calling. Ill try to post a schematic to see if that helps make my babbling make any sense.

EDIT: Schematic attached. Here is what the near-oil boiler piping looks like currently. My two circs are existing, on the return side of things, pumping towards the oil boiler. So cold water from the house comes into the circs, through the oil boiler, and out to the loads (through existing flow checks). It looks like this might be the Tarm "Solo4" piping layout.....

 

[hobbyheater]

A cast-iron slant fin oil boiler (295 lbs) with a 1.10 nozzle would run 3 1/2 hours in 24 hours to maintain standby reserve temperature. That represents a heat loss of 350,000 BTUs or about 50 lbs of wood in 24 hours.

Doesn't really sound right, not at all.

Back when oil was less than two dollars a gallon we ran an oil boiler all summer just to keep the boiler hot and dry and to supply DHW. I kept pretty good records and we burned less than a gallon a day, which is still a lot, but it's a whole lot less than three and half or four gallons a day

The oil boiler had a tankless coil so it had to maintain a constant temperature of no less than 175 F. The oil boiler was isolated from all demand and operated for three days. An hour meter was attached to the burner motor to record the time it ran and at the end of three days, the meter had recorded 10 1/2 hours. Stack loss was huge. When this experiment was done, furnace oil was $ 0.40 - $0.45 a gallon.
The next step involved plumbing the oil boiler direct to the 1000 gallon storage to get away from this stack loss and its oil consumption was reduced by around 60%.
About 6 years ago we switched to an electric boiler back up. We used the oil boiler backup so little that the furnace oil would gel.

 

[ewdudley]

The oil boiler had a tankless coil so it had to maintain a constant temperature of no less than 175 F. The oil boiler was isolated from all demand and operated for three days. An hour meter was attached to the burner motor to record the time it ran and at the end of three days, the meter had recorded 10 1/2 hours. Stack loss was huge. When this experiment was done, furnace oil was $ 0.40 - $0.45 a gallon.

Yikes, good thing you got to the bottom of what was going on, well done.

 

[ewdudley]

Indeed it is in my basement. I was thinking of heat loss up the chimney.

Just spoke with the installer, and he mentioned that he piped it this way deliberately. The reason being that if piped the opposite way, it would push through the flow checks and potentially heat all zones of the house when only one is calling. Ill try to post a schematic to see if that helps make my babbling make any sense.

EDIT: Schematic attached. Here is what the near-oil boiler piping looks like currently. My two circs are existing, on the return side of things, pumping towards the oil boiler. So cold water from the house comes into the circs, through the oil boiler, and out to the loads (through existing flow checks). It looks like this might be the Tarm "Solo4" piping layout.....

It's hard to believe that there would be enough pressure drop in the parallel flow through the boiler to overcome the existing forward flow resistance of the flow checks. If the wood boiler flow pumped into the supply side of the oil boiler, if one of the load pumps was off then the excess wood boiler flow would take the path of least resistance through the oil boiler.

You need on the order of one psi to overcome a flow check, is flow from circulator C-1 so large that the flow through the boiler would exceed one psi of pressure drop? Especially since one of the zones would be taking a substantial amount of flow to begin with.

Or if you really wanted to play it safe, you could connect wood boiler flow into the supply side of the oil boiler using the unused tappings as you originally proposed.

--ewd

 

[Clarkbug]

No idea Eliot.

I dont play with systems this size much, so Im trying to rely on the experience of others, and the installer guy said this is how he has been putting them in. At this point its late enough in the season Im really wanting him to finish up, and he will have all of the fittings in by tomorrow (hopefully). I want to know if I should have him swap the piping (after the circ, before the three way) which he will do if I tell him thats what I want. Or if I should just get it hooked up, start burning, and see what happens.

My brain says that I should have him switch it, but maybe Im missing something here? (Going to the tappings is really gonna slow things down, and I dont know my nerves can take that just yet....)

 

[ewdudley]

My brain says that I should have him switch it, but maybe Im missing something here? (Going to the tappings is really gonna slow things down, and I dont know my nerves can take that just yet....)

Yeah, switch it. You're right about the cool return mixing with the hot wood boiler supply and no good can come of that.

With fat pipes flowing through the boiler I can't see how you'd build up enough pressure to pop open a flow check. If worse comes to worse you could add a single big flow check on either the supply or return manifolds of the loads.

 

[Clarkbug]

Pipes on the boiler are 1 1/4" on either side, so not sure if they are fat enough, but I would hope that they are...